Respirable silica particles are well documented agents for the induction of lung diseases following inhalation. However, epidemiologic studies of respirable silica exposures, especially in mixed dust atmospheres, and pneumoconiosis or pulmonary fibrosis show regional or geological formation-specific variations in the correlation of exposure and disease prevalence.
In the United States and elsewhere the silica content of respirable dusts in occupational settings is usually measured for research or regulatory purposes by infrared spectroscopic or X-ray diffraction analysis on bulk samples of filter collected respirable dusts. This provides a mass percent silica value. Scanning electron microscopy X-ray spectrometry (SEM-XRS) is sometimes used for a particle by particle composition analysis. These methods of analysis typically give comparable data, and all fail to explain the variations in exposure and disease seen, for example, between different mining commodities or geologic formations. These exposure measures do not assay the biologically available active fraction of the silica dust; that is, they do not distinguish finely coated silica particles from bare silica surfaced particles, or account for the persistance or lack of persistance of such occlusions under conditions existing on the surface of the lung airways and air sacks.
Mining and some other dusty workplace atmospheres are monitored for respirable dust and quartz dust for purposes of occupational health protection. Respirable quartz exposure data are also acquired for epidemiological research on the correlation of pulmonary disease prevalence with respirable dust composition and concentration. For these purposes quartz content often is measured on collected dust samples by X-ray diffraction or i.r. spectroscopy. Scanning electron microscopy-energy dispersive X-ray analysis spectrometry (SEM-EDX) has also been used.
X-ray diffraction and i.r. spectroscopy methods measure the bulk composition of a multi-particle dust sample; SEM-EDX measures the elemental composition of single particles. These methods as typically applied, do not distinguish homogeneous distribution of impurities in a particle from possible heterogeneous surface distribution. These measurements of quartz content are not always accurate predictors of the prevalence of dust-induced disease, which implies that the biologically available quartz surface may not be equivalent to conventionally measured quartz content of respirable dust in some exposure settings.
Auger spectrometry and X-ray photoelectron spectrometry can be used to analyze the elemental composition of material surfaces, for surface layer thicknesses from one atomic layer to about 0.001 to 0.01 micrometers deep. However, to distinguish the surface from the underlying material, the surface, after analysis, must be destructively ion-etched off and the underlying material then analyzed. This often requires a number of analyze-and-etch steps. Also, incidental contamination, e.g., carbon from handling or solvents, can mask the surface of interest, requiring careful handling or pre-etching to bare the true surface for analysis.
These problems are exacerbated if one is attempting to study specific components of mixed materials, e.g., quartz respirable-sized particles from a mixed dust atmosphere. In that case, it may be impossible to pre-identify particles of a selected type for detailed surface analysis.
The present invention provides the ability to non-destructively analyze composition with depth below the surface, permitting selection of particles or areas which are then subjected to detailed surface analysis. The method permits identification of differences in the elemental ratios which can distinguish homogeneous composition, top surface coating, and top and bottom surface coating for particles of total thickness between about 0.1 to 10 micrometers.